The present invention relates to a process for preparing molecular sieves of faujasite structure and to the pre-shaped molecular sieve bodies prepared according to the present process. More specifically, the present invention relates to a process for preparing molecular sieves of faujasite structure which comprises contacting a precursor gel with steam, optionally followed by caustic washing, said process making it possible to form a zeolite of faujasite structure in pre-shaped bodies.
The aluminosilicates known as zeolites are highly complex chemical structures which present different crystalline structures as a function of their composition. Although they occur naturally, zeolites nowadays are mostly produced by industry aiming at various uses, the more important among which are application as adsorbents and as catalysts in the oil industry.
Among the various kinds of synthetic zeolites used in the oil industry, the one most frequently used is the zeolite of faujasite structure, which after its synthesis may show a molar composition according to the formula below:
0.9xc2x10.2 M2 O.Al2O3 x SiO2.w H2O 
wherein M represents a cation of an alkaline metal, x is a number between 2.5 and 6, and w is a number between 6 and 9.
When preparing faujasite zeolites, the molar ratio between the oxides of aluminum and silicon is a further parameter which may be adjusted. As found in natural environments, the faujasite SiO2/Al2O3 molar ratio is between 2.5 and 4. Synthetic zeolites of faujasite structure of higher SiO2/Al2O3 molar ratio have been prepared, since it was found that structures with a higher silica content are more resistant to temperature and acids, situations which are usually encountered in the application of zeolites in the oil industry.
A faujasite structure zeolite used mainly in industry, and especially in the oil industry, is the type Y zeolite, where the SiO2/Al2O3 molar ratio is higher than 4.5. The molar ratio of zeolites for use in fluid catalytic cracking catalysts, for example, is at least 5.0. However, the preparation of zeolites having such higher SiO2/Al2O3 molar ratio presents several drawbacks, since generally those zeolites are obtained through processes of extended periods of crystallization, which require huge crystallization vessels and a sharp control of the overall production process to avoid impurities, namely, different crystalline phases, which harm the end product.
In spite of these drawbacks, various industrial processes for obtaining type Y zeolites are presently in use, so crucial is the importance of this material in the production of more active and more selective catalysts for the oil industry. One such process which widely employs the type Y zeolite in the composition of catalysts is the fluid catalytic cracking process.
Brazilian Patent BR 8402808, for example, teaches a process for preparing a high-silica, faujasite zeolite to be used in fluid catalytic cracking, where sodium silicate and aluminum sulfate are contacted under controlled conditions of pH and temperature, the crystallization periods varying between 0.5 and 12 hours. The product is a Y zeolite of SiO2/Al2O3 molar ratio 5.6.
More detailed research on the preparation of type Y zeolite will reveal that most processes still in use employ the same operation sequences detailed in the above Brazilian Patent, the only variations concerning the raw materials and the formulation of the recipes. One could even say that nothing novel is to be expected in this field.
It is therefore clear to the experts in the production of catalysts and adsorbents that there is an unfulfilled need to develop new processes for preparing a type Y zeolite, processes which are more selective in order to avoid forming impurities, more economical, and most of all, capable of producing Y zeolites having features not yet attained by the state-of-the-art preparation processes.
Experts in the field of zeolite preparation who keep up with the technical literature of the field will be aware of the technique of zeolite crystallization described by X. Wenyang et al. in Journal of Chemical Communication Vol. 10, 1990, pp. 755, where ZSM-5 type zeolites of high SiO2/Al2O3 molar ratio were obtained through the contact of a precursor gel with vaporized organic compounds. According to this process, the crystallization of the precursor gel (amorphous to X-rays) occurs through the transport of the organic compounds (amines) in the vapor phase. The process thus developed ensures that the synthesis of these kinds of zeolites, which normally is effected using the precursors as an aqueous suspension and in the presence of a template agent, normally a quaternary ammonium salt or amines, is rendered simpler and more economical.
By applying the same technique, other authors effected the synthesis of zeolites of the ZSM family such as ZSM-21 and ZSM-35. Chinese Patent 1,051,334 teaches the preparation of various zeolites of the ZSM family.
M.-H. Kim, H.-X Li, and M. Davis in Microporous Materials Vol. 1, 1993, pp. 191-200, also comment on the preparation of ZSM-5 zeolite using the above-cited preparation process. In their work they compare the state-of-the-art technique in an aqueous medium with the new technique, including the use of steam, confirming that such technique can be used successfully for preparing that particular zeolite family.
In spite of the extensive work of Kim et al. exploring a wide range of compositions of the amorphous precursor, where the SiO2/Al2O3 molar ratio is varied between 2.5 and 80, the synthesis of type Y zeolite could not be achieved.
One single published work describes how to obtain a faujasite structure zeolite. J. Dong and P. Dong in Shiyou Huagong Vol. 24, 1995, pp. 321-324, comment on a type X zeolite of low SiO2/Al2O3 molar ratio. This product, however, is not attractive for use in the preparation of catalysts.
In order to make clear why the method proposed by X. Wenyang et al. leads more easily to the ZSM-5 zeolite than to the type Y zeolite, the differences between the two zeolites should be emphasized.
ZSM-5 zeolite, besides using a template agentxe2x80x94generally an ammonium quaternary saltxe2x80x94shows a much higher SiO2/Al2O3 molar ratio than the molar ratios usually encountered for type Y zeolites. This causes the excess silica found after the crystallization of the ZSM-5 zeolite to be very low compared to that observed for the synthesis of type Y zeolite. And it is exactly the excess silicaxe2x80x94that is, the amount of silica which is not incorporated into the crystalline network of Y zeolite during the crystallization stepxe2x80x94that should be controlled during the process of transforming an amorphous solid by the crystallization method, which involves contact with steam or organic compounds.
Another important feature in obtaining new kinds of catalysts and adsorbents is the forming of a type Y zeolite in pre-shaped bodies. A feature of this kind of material is that the surface of the pre-shaped bodies is covered with zeolite crystals. This higher concentration of zeolite in the outer layers of the pre-shaped body favors adsorption and catalysis.
Examples and references on the preparation of faujasite zeolites as pre-shaped bodies are scarce.
M. Matsukata and E. Kikuchi in xe2x80x9cZeolitic Membrane Synthesis, Properties and Prospects,xe2x80x9d Bulletin of the Chemical Society of Japan 70, pp. 2341-2356 (1997) obtained membranes of pentasil-type zeolites with the technique of transferring organic amine vapor onto gels of silica-rich compositions.
U.S. Pat. Nos. 3,657,154, 4,235,753, and 4,493,902 teach the preparation of a type Y faujasite zeolite as pre-shaped microspheres. These patents teach the preparation of type Y zeolites from kaolin microspheres. According to the process, the microspheres are calcined at elevated temperature, leading to meta-kaolin.
After this step, the meta-kaolin microspheres are immersed in an aqueous solution which contains ingredients such as sodium silicate and soda. Next, the suspension of meta-kaolin microspheres is heated to between 80 and 100xc2x0 C. and agitated slightly to avoid sedimentation and secure the homogenization of the medium to obtain type Y zeolite.
The preparation of type Y zeolite as pre-shaped bodies from kaolin is also the object of U.S. Pat. No. 3,4459,645, which teaches the preparation of type Y zeolite as extrudates. As in the case of microspheres, the calcination of kaolin extrudates to obtain meta-kaolin is a fundamental step in the preparation of type Y zeolite as extrudates.
It is therefore obvious to the experts that the processes taught in the above patents necessarily require the calcination of the pre-shaped kaolin bodies to obtain meta-kaolin, which not only constitutes the raw material of the type Y zeolite but also is of paramount importance for securing the physical integrity of the pre-shaped bodies, which otherwise would collapse through loss of their shape during the step of heating and agitation in the aqueous medium.
In the processes taught in the above cited patents, the zeolite is obtained only when the pre-shaped body is suspended with other raw materials such as sodium silicate and soda. That is why there is a homogenous distribution of the type Y zeolite over the pre-shaped bodies, as presented by J. K. Lampert et al., xe2x80x9cFluid Catalytic cracking catalyst micostructure as determined by a scanning ion microprobexe2x80x9d in Applied Surface Science, 55 (1992) 149-158. See especially the Conclusion on page 157.
The preparation of crystalline Y zeolite from a reaction mixture comprising silica and alumina and possibly a xe2x80x9cseedxe2x80x9d crystal is disclosed in U.S. Pat. No. 5,785,944, but the reaction mixture (precursor gel) requires a silica to alumina ratio (SAR) of from 4-7, preferably 4.5-6.
Advantageously, the preparation of type Y faujasite zeolites as pre-shaped bodies according to the present invention overcomes various drawbacks of the state-of-the-art technique directed to methods of zeolite preparation. Formation of a faujasite zeolite as pre-shaped bodies would avoid modifications of the zeolite properties during the molding processes, besides rendering the overall molding process easier. This is because highly crystalline materials (such as the zeolites), as opposed to amorphous or less crystalline materials which are easily molded, do not offer binding properties and require more complicated techniques and molding methods.
Thus, in spite of the state-of-the-art techniques for applying the method for preparing faujasite zeolites developed by X. Wenyang et al., so far there are no known techniques for preparing a type Y zeolite in high purity and yield as pre-shaped bodies, so as to provide the zeolite and catalyst industry with a novel process for preparing type Y zeolite as well as with novel type Y zeolite-containing products, where the type Y zeolite is prepared by contacting a solid precursor with steam, as is described and claimed in the present invention.
Broadly speaking, the present invention relates to a process for preparing a type Y faujasite zeolite which comprises the following steps:
preparing a precursor gel having from a source of silica-alumina or a source of silica and a source of alumina and an inorganic template agent or seed, the SiO2/Al2O3 ratio being adequate for forming a type Y faujasite zeolite in the sodium form, but in any event, greater than 7;
drying the precursor gel of type Y zeolite, optionally combined with shaping;
recovering the dried precursor gel as a powder or shaped body;
steam treating the dried precursor gel of type Y zeolite with steam under adequate conditions of temperature and time, optionally followed by a washing step;
optional washing of the dried and steamed precursor gel of type Y zeolite with the aid of an aqueous dilute caustic solution to remove any excess silica;
drying the type Y zeolite.
The present invention thus provides a process for preparing a faujasite type Y zeolite in high yield and crystalline purity and makes it possible to prepare the type Y zeolite as pre-shaped bodies as a desired objective.
The present invention also provides a process for preparing a type Y faujasite zeolite from the steam treatment of the precursor gel of type Y zeolite as pre-shaped bodies made up of an inorganic oxide-containing matrix and, optionally, catalytically active metals.
Further, the present invention provides pre-shaped bodies of type Y faujasite zeolite where the outer surface is coated with zeolite crystals from the steam treatment of the type Y precursor gel, thus making the contact of the reagents with the zeolite easier.